1. Field of the Invention
This invention relates to charged particle beam systems and more specifically relates to charged particle lithography tools for semiconductor manufacturing.
2. Description of the Related Art
Charged particle beam lithography is a well-known process for patterning in the fabrication of semiconductor integrated circuits by xe2x80x9cdirect writingxe2x80x9d. This form of lithography is also used for patterning in the fabrication of photolithography masks. Charged particle beam lithographic tools typically include a beam source, beam steering and forming elements, a stage for a semiconductor wafer or mask blank (the workpiece), and a mechanism to move the stage with respect to the beam. For electron beam lithography, the beam source is an electron source and the beam steering and forming elements are an electron optical column. One commercially available electron beam lithography system is the Etec Systems, Inc. MEBES(copyright) (xe2x80x9cManufacturing Electron Beam Exposure Systemxe2x80x9d). For focused ion beam lithography, the beam source is an ion source and the beam steering and forming elements are ion deflection and focusing elements.
Two common approaches to writing in an electron beam lithography system are referred to as xe2x80x9craster-scanxe2x80x9d and xe2x80x9cvector-scanxe2x80x9d. In raster scanning, an electron beam is scanned back and forth across the surface of the workpiece. The beam is turned on and off at appropriate times to create the desired pattern in an electron sensitive resist layer. The length of the scan distance on the substrate workpiece is typically limited to about 500 micrometers. To create patterns over large areas, the stage holding the workpiece is continuously moved. Both the beam scanning and the stage movement can introduce errors affecting the quality of the exposed patterns.
A vector scan electron beam lithography system operates in much the same way except that the beam is deflected only to positions at which pattern elements are to be exposed. The individual pattern elements are often written in raster fashion. During writing, the stage is generally stationary, and writing takes place over only a limited field, typically square in shape. Once the writing of the field is completed, the stage is moved to a new location, and another field written. Frequently, vector scan uses a variable shaped beam, which is a beam capable of having a different plan view size and/or shape each exposure. The pattern is then composed from these variable shapes. A shaped beam is capable of exposing multiple pixel sites simultaneously instead of one pixel site at a time, as in a raster scan strategy.
The tuning, diagnosis, and qualification of electron beam lithography systems is typically a lengthy process involving the writing and observation of a pattern of test patterns (features) on a workpiece. From these test patterns, placement errors can be identified, which assists in specifying setup parameters and diagnosing problems. In one exemplary process called MARKIT(copyright), a pattern, typically a symmetric array of crosses, is written onto the workpiece. The workpiece is then rotated and the locations of the marks after development are measured. This allows detection of deviations from symmetry induced by the machine. From these measurements, a map of deviations from intended positions is produced, and analysis of this map allows problems to be diagnosed. One cycle of the procedure may take from hours to days to complete, so the time required for a feedback loop of diagnosis, repair, verification, and qualification can seriously impact a manufacturing facility""s productivity.
In addition to the time required for these prior art techniques, there are various other disadvantages as well. First, the number of usable marks is limited, so marks tend to be large and widely spaced on the workpiece. Disturbances with a high spatial frequency may be overlooked during the inspection process because of the limited coverage of the test patterns. In addition, some periodic effects may be masked by the periodicity of the test patterns or by drift during measurement. Measurement precision should be in the range of approximately 10 nm, so there is often substantial scatter in the data. It may also be difficult to interpret temporal behavior from its consequences in the space domain due to aliasing and sampling effects. Finally, using this technique, it can be difficult to isolate error sources, particularly those that have their origin in asynchronous beam or stage vibrations.
U.S. Pat. No. 5,808,731 to Joseph P. Kirk describes a system and method for visually determining the performance of a photolithography system. Using this method, two patterns having different spatial frequencies are formed onto a workpiece. Each of these patterns individually include features small enough to make them difficult to optically resolve when using light in the visible regions. Therefore, in order to inspect the patterns, a scanning electron microscope (SEM) or similar inspection tool must be used to resolve such fine features. However, when the two patterns are overlaid, they combine to form a pattern in the resist that exhibits Moire beats. The Moire beat spatial frequency of the resulting pattern is significantly lower than the two individual spatial frequencies used, and thus can be visually inspected using light in the visible regions. Although Kirk teaches a method which enables the inspection of very fine features, it requires two lithography processes (including exposure and development), which results in significant delays, as described above. It also requires a second instrument (SEM).
xe2x80x9cBeam-on-edgexe2x80x9d techniques have also been used to detect displacement errors in electron beam lithography tools. A Gaussian or shaped beam is directed onto a special test transmission grid, and the current passing by the edge is detected. If the beam is directed such that half of the beam is occluded by the edge, then the transmitted current detected is one half of the total beam current, I0. If the beam or stage experiences unwanted motion or placement error, then the error signal, Ie=xcex94I/I0, is proportional to the placement error amplitude divided by the beam size (xcex94x/x or xcex94y/y). Because the beam is comparable to the expected size of errors, the error signal for very small motions is a substantial fraction of the total current.
A disadvantage of current xe2x80x9cbeam-on-edgexe2x80x9d methods is that they require a transmission grid/detector and a stationary stage. Thus, these methods cannot be used to measure accuracy while the beam or stage are moving. They also cannot be used on the workpiece itself because the electron beam cannot pass through to the transmission detector.
Accordingly, an improved system for calibrating and troubleshooting charged particle beam lithography equipment is needed. In particular, it would be desirable to have a system which decreases the cycle time for testing and analyzing, and which can be used to diagnose tools during their normal writing operations.
In accordance with the present invention, a method for diagnosing errors in a charged particle beam system comprises patterning a workpiece using a charged particle beam to expose a test pattern having a plurality of features, processing the pattern, directing a charged particle beam at said workpiece in an overlay pattern using a similar exposure sequence, receiving a signal generated by an interaction of said workpiece and said charged particle beam, and analyzing said signal.
In accordance with another aspect of the present invention, a method for diagnosing errors in a charged particle beam system comprises patterning a workpiece with a test pattern using a charged particle beam tool, directing a first charged particle beam from said charged particle beam tool at said workpiece in an overlay pattern, receiving a first signal generated by an interaction of said workpiece and said charged particle beam, and recording said first signal. Then, a second charged particle beam is directed from said charged particle beam tool at said workpiece in said test pattern. A second signal generated by an interaction of said workpiece and said charged particle beam is received and this second signal is compared with the first signal.
In accordance with yet another aspect of the present invention, a charged particle beam system comprises a charged particle beam source for providing a charged particle beam, a stage, a workpiece provided on said stage, said workpiece being patterned with a first test checkerboard pattern of repeating square features, a charged particle beam column for directing said charged particle beam onto said workpiece, and a backscatter detector for detecting a signal generated by an interaction of said charged particle beam and an edge of each of said features in said test pattern.
In accordance with another aspect of the present invention, a standard pattern substrate for testing a charged particle beam tool comprises a substrate having an upper surface, a coordinate system defined by an x-direction and a y-direction orthogonal to said x-direction, said x-direction and said y-direction defining a plane parallel to said upper surface of said substrate, and a plurality of metallic features patterned on said upper surface of said substrate, each of said metallic features having a first edge parallel to said x-direction and a second edge parallel to said y-direction.